1. Field of the Invention
The present invention relates to image processing apparatuses, image processing methods, and cameras, and more particularly, to an image processing apparatus and an image processing method for processing the output signal of a solid-state imaging device having a color filter with the specified color arrangement on a light receiving plane and a camera using the image processing apparatus.
2. Description of the Related Art
In an image processing apparatus for processing the output signal of a solid-state imaging device having a color filter with, for example, a Bayer color arrangement of primary colors on a light receiving plane, since each color is located not continuously on the color filter, interpolation processing is performed so as not to reduce the impression of resolution. In a conventional image processing apparatus of this type, correlation values are calculated only in two directions, such as the two opposite directions (right and left directions) in the horizontal direction or the two opposite directions (upper and lower directions) in the vertical direction, and interpolation is performed according to the calculated correlation values. In other words, whether horizontal stripes or vertical stripes are viewed is determined from information of pixels around a pixel to be interpolated, for example, five by five pixels around the pixel to be interpolated. In a case in which horizontal stripes are viewed, interpolation is performed by the use of the signals of the pixels located at both sides in the horizontal direction of the pixel to be interpolated. In a case in which vertical stripes are viewed, interpolation is performed by the use of the signals of the pixels located at both sides in the vertical direction of the pixel to be interpolated.
Although the conventional image processing apparatus, which performs interpolation with the use of correlation detection only in two directions, is effective for edges in the horizontal and vertical directions, it cannot detect the intersection of a horizontal edge and a vertical edge, or a correct edge at a broken or bent segment, if the edges are about one pixel wide. As for a diagonal line, the apparatus cannot determine whether horizontal correlation or vertical correlation is strong and is likely to average (perform average interpolation) the results of processing at a horizontal edge and processing at a vertical edge.
Therefore, the conventional image processing apparatus causes some problems, such as a blurred image with an impression of low resolution or a missing line at a place where it should not be, at the portions described above.
Accordingly, it is an object of the present invention to provide an image processing apparatus and an image processing method which allow successful interpolation processing to be performed even at a broken or bent line and a right-angled corner, and a camera using the image processing apparatus.
The foregoing object is achieved in one aspect of the present invention through the provision of an image processing apparatus for processing the output signal of a solid-state imaging device having a color filter with the specified color arrangement on a light receiving plane, including: a correlation-value calculation circuit for calculating, according to four pieces of pixel information in four directions which form angles multiples of 90 degrees with each other, the right, left, upper, and lower directions, from a pixel to be interpolated, correlation values in the four directions; a coefficient determination circuit for determining interpolation coefficients according to the correlation values in the four directions calculated by the correlation-value calculation circuit; and an interpolation processing circuit for performing interpolation according to the four pieces of pixel information corresponding to the four directions with the use of the interpolation coefficients determined by the coefficient determination circuit.
The foregoing object is achieved in another aspect of the present invention through the provision of an image processing method for processing the output signal of a solid-state imaging device having a color filter with the specified color arrangement on a light receiving plane, comprising the steps of: calculating, according to four pieces of pixel information in four directions which form angles multiples of 90 degrees with each other, the right, left, upper, and lower directions, from a pixel to be interpolated, correlation values in the four directions; determining interpolation coefficients according to the calculated correlation values in the four directions; and performing interpolation processing according to the four pieces of pixel information corresponding to the four directions with the use of the determined interpolation coefficients.
In the image processing apparatus and the image processing method, characteristic values, namely, correlation values which indicate the degrees of correlation, of a pixel to be interpolated in a total of four directions, the right and left directions (horizontal direction) and the upper and lower directions (vertical direction), are first calculated. Then, according to the correlation values in these four directions, interpolation coefficients used for interpolation are determined. The four pieces of pixel information in the four directions, which are used for calculating the correlation values, are multiplied by the interpolation coefficients and summed up to implement interpolation processing.
The foregoing object is achieved in yet another aspect of the present invention through the provision of a camera including: a solid-state imaging device having a color filter with the specified color arrangement on a light receiving plane; an optical system for forming an image of an object by incident light on the light receiving plane of the solid-state imaging device; and an image processing apparatus for processing the output signal of the solid-state imaging device, the image processing apparatus having: a correlation-value calculation circuit for calculating, according to four pieces of pixel information in four directions which form angles multiples of 90 degrees with each other, the right, left, upper, and lower directions, from a pixel to be interpolated, correlation values in the four directions; a coefficient determination circuit for determining interpolation coefficients according to the correlation values in the four directions calculated by the correlation-value calculation circuit; and an interpolation processing circuit for performing interpolation according to the four pieces of pixel information with the use of the interpolation coefficients determined by the coefficient determination circuit.
As described above, according to the present invention, when the output signal of a solid-state imaging device having a color filter on a light receiving plane is processed, since, according to four pieces of pixel information in four directions, the right, left, upper, and lower directions, from a pixel to be interpolated, correlation values are calculated in the four directions; interpolation coefficients are determined according to the calculated correlation values in the four directions; and interpolation processing is performed according to the four pieces of pixel information with the use of the determined interpolation coefficients, the degrees of correlation can be successfully determined even for an edge disposed in a direction not perpendicular to any of the directions for which the correlation values are calculated. Therefore, a diagonal edge is successfully shown without any blur after interpolation processing and successful interpolation is performed at a broken or bent line and a right-angled corner.